1. Field of the Invention
The present invention relates to a magnetic sensor system for detecting a physical quantity associated with the relative positional relationship between a scale and a magnetic sensor.
2. Description of the Related Art
In recent years, sensor systems such as rotary encoders and linear encoders have been employed to detect a physical quantity associated with the rotational movement or linear movement of a moving object in a variety of applications. Such sensor systems typically include a scale and a sensor, and the sensor is configured to generate a signal associated with the relative positional relationship between the scale and the sensor. Among such sensor systems, one that uses a magnetic sensor as the sensor will be called herein “magnetic sensor system”. Magnetic sensor systems are disclosed in, for example, JP-H04-5571 A, U.S. Pat. No. 6,924,639 B2, and U.S. Pat. No. 7,078,892 B2.
The scale of the magnetic sensor system for use with a rotationally moving object is, in general, a rotating body that moves in response to the movement of the moving object. The rotating body can be, for example, a multipole-magnetized magnet having a plurality of pairs of N and S poles alternately arranged in a circumferential direction, or a gear having teeth formed of a magnetic material. In this case, the magnetic sensor system detects, for example, the rotational position and/or the rotational speed of the rotating body as the physical quantity.
The scale of the magnetic sensor system for use with a linearly moving object is, for example, a linear scale having a plurality of pairs of N and S poles arranged alternately in a linear configuration. In such a case, one of the linear scale and the magnetic sensor moves in response to the movement of the moving object, and the magnetic sensor system detects the relative position and/or speed of the linear scale with respect to the magnetic sensor as the physical quantity.
In the magnetic sensor system, continuous changes in the relative positional relationship between the scale and the magnetic sensor cause periodic changes in the direction of a magnetic field at a certain point in the magnetic sensor. Herein, the amount of a change in the relative positional relationship between the scale and the magnetic sensor that changes the direction of the magnetic field at a certain point by one period is referred to as one pitch.
Some known rotary encoders and linear encoders are configured so that the sensor includes first and second detection units disposed at mutually different positions and the two detection units generate a first signal and a second signal that have mutually different phases. This configuration makes it possible to detect the direction of relative movement of the scale with respect to the sensor by determining whether the phase of the second signal is advanced or delayed with respect to the phase of the first signal.
Now, a description will be given concerning the problem to be raised when the magnetic sensor system is configured so that the magnetic sensor includes the first and second detection units mentioned above. In this case, each of the first and second detection units includes a magneto-sensitive element. As disclosed in U.S. Pat. No. 6,924,639 B2 and U.S. Pat. No. 7,078,892 B2, many of conventional magnetic sensor systems employ an anisotropic magnetoresistive element as the magneto-sensitive element. The anisotropic magnetoresistive element requires a relatively large footprint. Thus, to employ the anisotropic magnetoresistive element as the magneto-sensitive element, the distance between the first detection unit and the second detection unit must be large to some extent. This leads to the problem that the first detection unit and the second detection unit tend to have mutually different detection characteristics due to the following various factors. One factor is that the first detection unit and the second detection unit may be subjected to mutually different noise magnetic fields. Another factor is that the physical distance between the first detection unit and the scale may differ from the physical distance between the second detection unit and the scale. Still another factor is that the effective distance between the first detection unit and the scale may differ from the effective distance between the second detection unit and the scale due to, for example, adhesion of magnetic metal powder to the scale.
Further, in the case of the anisotropic magnetoresistive element, a decrease in the magnitude of one pitch would make it difficult to accurately detect a change in the magnetic field caused by a change in the relative positional relationship between the scale and the magnetic sensor.
JP-1104-5571 A discloses a rotation detector including: a rotating magnet having only a pair of N and S poles; first to third magnetoresistive elements disposed at intervals of 120° on the same circumference about the rotation axis of the magnet; a first differential operational amplifier for computing the difference between the output of the first magnetoresistive element and the output of the third magnetoresistive element; and a second differential operational amplifier for computing the difference between the output of the second magnetoresistive element and the output of the third magnetoresistive element.
The above-described problem holds true for this rotation detector because the positions of the first to third magnetoresistive elements are significantly different from each other. Further, since the first to third magnetoresistive elements are large, it is difficult to apply the technique disclosed in JP-H04-5571 A to a magnetic sensor system that uses, as the scale, a multipole-magnetized magnet having a plurality of pairs of N and S poles arranged alternately in a circumferential direction.
As can be seen from the foregoing, heretofore there have not been provided any magnetic sensor system that allows a change in the relative positional relationship between the scale and the magnetic sensor by two pitches or more and that is able to detect, with high accuracy, a physical quantity associated with the relative positional relationship between the scale and the magnetic sensor, including the direction of relative movement of the scale with respect to the magnetic sensor.